The invention relates to a radiation detection device, comprising a housing which comprises an entrance window and in which there is arranged at least one ionization chamber detector which comprises a flat high-voltage electrode which is directed transverse to the entrance window and a flat collector electrode which is arranged to be substantially parallel to the high voltage electrode. The collector electrode has an end face which faces the entrance window and which is electrically connected to a contact face which is situated opposite the end face and which forms part of a flat auxiliary electrode which extends parallel to the entrance window.
A radiation detection device of this kind is particularly suitable for use in a computer tomography apparatus in which a body is irradiated from a large number of directions by means of a fan-shaped radiation beam. A radiation detection device comprising a large number of detectors serves to measure the radiation absorption of the body along a large number of radiation paths. On the basis of the measured data, the density distribution of the irradiated part of the body is calculated and displayed on, for example, a television monitor by means of a computer.
German Patent Application No. 26.09.626 (which corresponds to U.S. Patent application Ser. No. 774,135, filed Mar. 3, 1977, now abandoned) discloses a radiation detection device of the described kind in which the auxiliary electrode is made of an electrically conductive material extending to the vicinity of the high-voltage electrode without electrically contacting it. This auxiliary electrode intercepts charge carriers formed by ionization which, in the case of a detector without such an auxiliary electrode, would be incident on the entrance window instead of on the collector electrode so that they would not contribute to a detector output signal. Because the auxiliary electrode is electrically connected to the collector electrode, the charge carriers intercepted by the collector electrode do contribute to the detector output signal. As a result, the detection efficiency of the detector is high in comparison with a detector which does not include such an auxiliary electrode. Radiation which enters the detector via the entrance window produces a comparatively large number of charge carriers in the immediate vicinity of the entrance window, because the intensity of the radiation, and the associated number of ionizations caused thereby, exponentially decreases as a function of the distance from the entrance window in the direction transverse to the window.
In the known detector, the distance between the auxiliary electrode and the high-voltage electrode is smaller than the distance between the collector electrode and the high-voltage electrode. As a result, after application of a high voltage between the electrodes, the electric field arising between the auxiliary electrode and the high-voltage electrode will be larger than the electrical field arising between the collector electrode and the high-voltage electrode. The electric field in the detector thus exhibits a spatial inhomogeneity, with the result that the detector is comparatively slow, because the measuring speed, being proportional to the drift speed of charge carriers formed by ionization, is limited in the case of high electric field strengths by space charges caused by avalanches of secondary ionizations and in the case of low electric field strengths by the value of the field strength.